autodoc: Add Optional[t] to annotation of function and method

As typing.get_type_hints() doing, this adds Optional[t] to type
annotations if a default value equal to None is set.

Note: this is default behavior of inspect.signature() since Python 3.10.

tests/test_ext_autodoc_configs.py

@@ -490,7 +490,7 @@ def test_autodoc_typehints_signature(app):
         '.. py:module:: target.typehints',
         '',
         '',
-        '.. py:class:: Math(s: str, o: Any = None)',
+        '.. py:class:: Math(s: str, o: Optional[Any] = None)',
         '   :module: target.typehints',
         '',
         '',